A typical battery operated device, such as a camera, a music player, or other device, may have an operating voltage of, for example, 3.3 volts. The battery used for the device may have a fully charged voltage of, for example, 4.7 volts and eventually discharges down to 2.7 volts over time. A voltage regulator internal to the device regulates the battery voltage to provide a constant operating voltage of 3.3 volts in this example.
A common regulating technique is pulse width modulation (PWM), where a switching transistor switches on and off at a fixed frequency, but at an adjusted duty cycle, so that the average current equals the load current at the desired regulated voltage. The pulses are smoothed by a filter to deliver a DC output voltage. Other PWM regulators use a variable switching frequency.
It is known to provide a combined buck/boost regulator that either steps down or steps up the battery voltage, as needed, to supply the constant operating voltage. Such buck/boost regulators typically compare the input voltage to the output voltage to determine whether to enter the buck mode or boost mode of operation. Using such a technique results in loop instability around the transition point.
Other techniques used for transitioning between buck and boost modes use differential input and output operational amplifiers, which are difficult to stabilize.
What is needed is a robust technique for transitioning between a buck mode and boost mode of a PWM regulator that does not suffer from the drawbacks of the prior art.